A. Field of the Invention
The field of the present invention is wireless (radio) communications. In particular, the field is using antenna arrays and spatial signal processing in wireless communications systems to perform demodulation, including correction for frequency offset and alignment, in the presence of co-channel interference.
B. Background
Spatial processing
Users of a wireless communications system typically access the system using remote terminals such as cellular telephones and data modems equipped with radio transceivers. Such systems generally have one or more radio base stations, each of which provides coverage to a geographic area known as a cell. The remote terminals and base stations have protocols for initiating calls, receiving calls, and general transfer of information.
In such a system, an allocated portion of the spectrum is divided up into communication channels which may be distinguished by frequency, by time, by code, or by some combination of the above. Each of these communication channels will be referred to herein as a conventional channel. To provide full-duplex communication links, typically some of the communication channels are used for communication from base stations to users' remote terminals (the downlink), and others are used for communication from users' remote terminals to base stations (the uplink). Within its cell, a radio base station can communicate simultaneously with many remote terminals by using different conventional communication channels for each remote terminal.
We have previously disclosed spatial processing with antenna arrays to increase the spectrum efficiency of such systems. See U.S. patent applications: Ser. No. 07/806,695 filed Dec. 12, 1991, entitled Multiple Access Wireless Communications Systems (also U.S. Pat. No. 5,515,378 issued May 7, 1996); Ser. No. 08/234,747, filed Apr. 28, 1994, entitled Method and Apparatus for Calibrating Antenna Arrays (now U.S. Pat. No. 5,546,909 issued Aug. 13, 1996); Ser. No. 08/283,470, filed Aug. 1, 1994, entitled Spectrally Efficient and High Capacity Acknowledgment Radio Paging System now U.S. Pat. No. 5,625,880; and Ser. No. 08/375,848, filed Jan. 20, 1995, entitled Spectrally Efficient High Capacity Wireless Communications Systems now U.S. Pat. No. 5,592,490 (collectively, "Our Co-Pending Patent Applications"). The general idea is to increase the quality of communication by using an antenna array rather than a single antenna, together with processing of the signals received at the antennas. The antenna array also can be used to increase spectrum efficiency by adding spatial multiplexing to conventional channels so that several users can communicate simultaneously on the same conventional channel. We call this SDMA for spatial division multiple access. Thus, taking frequency division multiplexing (FDMA) as an example, with SDMA, several remote terminals may communicate with one or more base stations on a single cell on the same frequency channel, that is, on the same conventional channel. Similarly, with time division multiplexing (TDMA) and SDMA, several remote terminals may communicate with one or more base stations on a single cell on the same frequency channel and the same time slot, that is, on the same conventional channel. SDMA likewise also can be used with code division multiple access (CDMA).
The general problem addressed by the present invention is to design a wireless communication system to be able to successfully receive and demodulate a particular signal or signals from a particular source in the presence of interfering signals from one or more interfering sources. In many situations, in particular in the case of cellular communications systems, the interfering signals are actually from other sources in the same communications system and so have the same modulation format. Such interference is one of a variety of possible interference from other signals on the same channel, so is called co-channel interference. The present invention addresses demodulating a signal in the presence of such co-channel interference as well as other interference and noise. A figure of merit by which one can evaluate such a system is how well one can pick up the desired signal, compared to the strength of the interference sources.
As in Our Co-Pending Applications, the present invention augments a wireless communication system with multiple antennas, thereby introducing multiple versions of each signal, each of these versions comprising the composite of all the co-channel signals together with interference and noise. With multiple antennas, the relationship in both amplitude and phase of a signal of interest to the interfering co-channel signals will be different in each of the antenna signals (each of the m signals in an m antenna system) due to geometric considerations, both because the antennas are separated by some distance, and, in some cases, because the different sources also are separated. In application to cellular communication systems, the use of multiple receiving antennas is predicated upon the fact that the various base stations' antennas are not co-located, nor are the sources.
Spatial processing of the (complex valued) m signals at the m antennas comprises for each signal of interest determining a weighted sum of the antenna signals. The complex valued weights can be represented by a vector called herein a weight vector. The more general situation is that the received antenna signals need also to be temporally equalized, and in that situation, rather than a weighted sum, for each signal of interest, a sum of convolutions of the antenna signals is determined. That is, the weight vector is generalized, for the linear time invariant equalization situation, to a vector of complex valued impulse responses. For the purposes of this invention, the term weight vector shall apply either to a vector of complex weights or to a vector of impulse responses, depending on whether temporal equalization is included.
Several techniques, including some of the techniques described in Our Co-Pending Patent Applications, have been proposed for receiving signals in the presence of co-channel interference using antenna arrays and using available or estimated spatial information. The method of the present invention does not require prior spatial knowledge, but exploits temporal information, in particular the modulation format of the incoming signal. Exploiting the modulation format in the presence of interfering signals of other modulation formats is relatively easy and there are many known methods that do this. The method of the present invention exploits the fact that the signal of interest has a particular modulation format, and works not only in the presence of such interfering, but also in the presence of interfering signals which have the same modulation format. That is, when there is also co-channel interference.
Prior art techniques do exist that separate and demodulate signals in the presence of co-channel interference and that exploit the fact that one has a particular modulation format. They have been proposed in published papers, for example: A. van der Veen and A. Paulraj, "A constant modulus factorization technique for smart antenna applications in mobile communications," in Proc. SPIE, "Advances Signal Processing Algorithms, Architectures, and Implementations V" (F. Luk, ed.), vol. 2296, (San Diego, Calif.), pp. 230-241, July 1994; S. Talwar and A. Paulraj, "Recursive algorithms for estimating multiple co-channel digital signals received at an antenna array," in Proc. Fifth Annual IEEE Dual Use Technologies and Applications Conference, May 1995; S. Talwar, M. Viberg and A. Paulraj, "Blind estimation of multiple co-channel digital signals arriving at an antenna array," in Proc. 27th Asilomar Conference on Signals, Systems and Computers, volume I, pp. 349-342, 1993; and A. L. Swindlehurst, S. Daas and J. Yang, "Analysis of a decision directed beamformer," IEEE Transaction on Signal Processing, vol. 43, no. 12, pp. 2920-2927, December 1995. As will be described more fully below, these published techniques may not work in practice because of implementation problems. That is, they do not tend to take into consideration the "real world" properties of signals.
These prior art techniques sometimes are called property restoral techniques because they force any estimates of signals of interest to have certain modulation formats or other structural properties that the actual signals are known to possess. For example, constant modulus techniques are known that use modulation schemes that have constant amplitude, and exploit that property. In addition to the implementation problems stated above, constant modulus techniques are not applicable to common modulation schemes that are not constant modulus, such as quadrature amplitude modulation (QAM).
The method and apparatus of the present invention also is property restoral, and is applicable to a very wide class of modulation schemes--those that have "finite alphabet." These are modulation formats in which the amplitude and phase of the signal at particular periods of time occupy one of some finite set of options. Many digital modulation techniques have this property. All the uncertainty in such a signal's value at any time is due only to synchronization and which symbol of the finite alphabet was transmitted. The preferred embodiment uses .pi./4 differential quaternary (or quadrature) phase shift keying (.pi./4 DQPSK), but the invention is applicable to any finite alphabet modulation.
A prior-art property restoral technique using an array of m antennas to give m received antenna signals from p original signals transmitted with a known digital modulation scheme is to recursively carry out the following steps to separate and demodulate a particular signal of interest:
a) Starting from some demultiplexing weight vector for the signal, form a new estimate of the signal of interest from the arriving antenna data; PA1 b) demodulate the new estimate of the signal to obtain an estimate of the transmitted symbols; PA1 c) from the estimate of the symbols transmitted, form a reference signal which is the closest estimate of the signal that was actually transmitted (that is the signal that has the known modulation format); and PA1 d) once one has a reference signal, determine the necessary spatial demultiplexing weight vector for the signal, that is, solve for that combination of the received signals at the antennas which most closely resembles the reference signal (step 1 again).
In this way, starting from some initial point, one recurses until one obtains a "very good" set of transmitted symbols and a "very good" set of spatial demultiplexing weights to apply to the antenna outputs to produce a "very good" estimate of the reference signal.
Prior art techniques that perform these steps include those of the references listed above. The recursion sometimes is called alternating projections in the literature because if one considers the set of demultiplexing weights as a complex valued vector w.sub.r, the recursion can be described as: starting with an estimate for w.sub.r, project this into reference signal space to get a better estimate of the reference signal, and project the better estimate of the reference signal into w.sub.r -space to get a better estimate of w.sub.r, and iterate back and forth between w.sub.r -space and reference signal-space until one obtains a "very good" w.sub.r that produces a "very good" estimate of the reference signal.
The starting value, either in w.sub.r space or in reference-signal space, first needs to be determined. As would be clear to one of ordinary skill in the art, either value is sufficient, because if one has a good guess at a reference signal, one can get a next better guess at w.sub.r, and conversely, if one has a good guess at a w.sub.r, one can generate a better guess at a reference signal. The prior art literature on alternate projection methods suggests that one could start with some estimate using such prior art methods as ESPRIT or MUSIC, and use this estimate as a starting point to the general recursion. There are other known ways to obtain a starting w.sub.r. For example, one can use well-known maximum ratio combining to get a starting w.sub.r, or well-known principal component copy techniques to get a starting w.sub.r. Using such techniques gives a starting w.sub.r that usually causes convergence upon the strongest signal. Thus, if the goal is to always pick out the strongest signal from a set of interferers, then such techniques work fine. However, such prior art techniques do not in general work well when one has a low carrier to interference ratio (C/I) as is the case when one has strong co-channel interference.
Our Parameter Estimation Invention discloses a technique for finding a starting w.sub.r estimate that extracts a signal that is not necessarily the strongest signal and that works well in the presence of strong co-channel interference.
In addition, prior art techniques for using a starting w.sub.r and then carrying out the alternating projections require, in order to work properly, that one first correct for any frequency offset and that one first align (synchronize) in time.
The frequency offset problem can be described as follows. In a typical radio-frequency (RF) receiver, the original RF signal is mixed down using local frequency references, typically produced by crystal oscillators and/or frequency synthesizers, to produce a baseband signal whose phase and amplitude changes around in a predictable pattern determined by the modulation format. Ideally, the signal has no residual frequency offset component, such an offset due for example to frequencies of the local oscillators differing slightly from the frequency of the oscillators used in sending the signals. In the case of mobile communications transmitting from a handset to a base station, the frequency of the radio signal is produced by a local oscillator in the hand set, while the frequency references used for down-converting the signal are produced by different local oscillators in the base station. Although the base station local oscillators typically are very good, there still typically is frequency offset in the residual signal.
The alignment problem is to synchronize exactly the initial timing of the symbols in the signals sent and the signals received in the base station. There are a number of techniques in the prior art for performing the alignment. Such techniques often use known training sequences that are incorporated in the burst of interest. These training sequences are chosen to have particular correlation (or convolution) properties. A correlation (or convolution) operation can then be used to determine timing offset, as is known in the art. The problem with such techniques is that they do not perform well in the presence of high co-channel interference.
Our Parameter Estimation Invention discloses technique for finding the starting time alignment and starting frequency offset that works well in the presence of strong co-channel interference.
In addition to the starting weight vector determining problem and initial alignment and frequency offset problem, prior art alternating projection methods also suffer from time alignment (synchronization) problems and frequency offset problems on an ongoing basis. First, the step of going from a reference signal to a next guess at w.sub.r is very sensitive to having the reference signal and the received signals at the antennas lined up correctly in time. If they are lined up incorrectly, then the w.sub.r one estimates may not be useful. In addition, in the step of projecting from a reference signal into a next better guess at a w.sub.r vector, there typically is a small frequency difference between the reference signal that one uses in the projections, and the frequency of the actual signals one is solving for. Such an offset can completely throw off the w.sub.r estimate. In the phase space, such small frequency differences gradually accumulate over time, so that after only a few symbols are transmitted, a large fraction of a cycle may be accumulated. Thus, the complex-valued solution for what phase one should apply to a particular signal gets totally thrown off. Thus, the straightforward solution of generating a new w.sub.r from a current reference signal is very sensitive to small frequency offsets.
Thus there is a need in the art for demodulation and signal separation techniques that are insensitive to the frequency offset and time alignment problems, and that work well in the presence of high level of co-channel interference. There is also a need in the art of improving alternating projection techniques by including time alignment and frequency offset estimation on an ongoing basis, such estimation working well in the presence of strong co-channel interference. Thus there is also a need in the art for improving the step of generating a reference signal (projecting onto signal reference space) in such alternating projections method, such improvement reducing the frequency offset and alignment problems in such reference signals.
Frequency offset is primarily a problem in finite alphabet modulation formats that include a phase difference between the symbols in the alphabet. This includes all phase shift keying (PSK) systems and many QAM systems. There are also modulation formats, including AM and QAM systems, which are sensitive to amplitude errors. In the alternating projections step for such systems, amplitude error creep may become a problem. That is, erroneous results may occur if one does not take into account the amplitude errors between a reference signal and the actual signal. Thus there is also a need in the art for improving the step of generating a reference signal (projecting onto signal reference space) in such alternating projections method, such improvement reducing the amplitude offset in such reference signals.
The method and apparatus of the present invention does not suffer from these problems. Our method includes projecting signals close to the actual, not ideal signals, in the presence the above described problems. The method is applicable to all finite alphabet modulation formats.
Producing better reference signals which are insensitive to frequency offset, time alignment and/or amplitude offset errors is applicable not only to alternating projection methods used in demodulation, but also to all processing of signals which require producing a reference signal, such as many adaptive filter processing, decision feedback equalization systems, etc.